1. Field of the Invention
The present invention relates to a film forming apparatus for forming a film of an adhesion layer on a substrate and more particularly to a film forming apparatus capable of forming a film of an adhesion layer having a sufficient adhesion and good permeability to a substrate by keeping the partial pressure of moisture low in the film forming apparatus.
2. Description of the Related Art
In existent film forming apparatus, when a thin film (for example, an optical thin film or an anti-reflection film) is formed on a plastic substrate, poor adhesion between the thin film and the plastic substrate often gives rise to a problem. As a method of solving the problem, it may be considered a method of increasing the adhesion strength by forming an adhesion layer for adhering a thin film to a plastic substrate, in which an SiO.sub.x film (x&lt;2) or the like is used as the adhesion layer.
However, in the existent film forming apparatus, it is difficult to stably form on SiO.sub.x film on the plastic substrate in the existent film forming apparatus. This is because SiO.sub.x has no stoichiometrically stable composition as in SiO.sub.2, so that x can not be defined constant easily and it is difficult to property control the amount of oxygen supplied to the plastic substrate upon film formation in order to make x constant. Further, the effect of the adhesion layer (adhesion strength) varies depending on the value for x, in which the adhesion is reduced as x approaches 2, whereas the adhesion is improved as x decreases. However, as x decreases, the transmittance of light of the SiO.sub.x film is lowered and the film is not suitable to the application use for the optical film.
In view of the foregoing situations, it may be considered the following method of stably forming an SiO.sub.x film with good transmission of light and adhesion. This is a method of observing the transmittance of light of a formed SiO.sub.x film upon forming the SiO.sub.x film on the plastic substrate in the film forming apparatus, judging based on the value whether the current flow rate of oxygen supplied by an MFC or the like to a plastic substrate is excessive or insufficient and controlling the flow rate of oxygen to be supplied depending on the result of the judgement.
Then, in order to control x for the SiO.sub.x film to an appropriate value, another factor should be taken into consideration. This is a requirement that the partial pressure of the moisture in a film-forming chamber be kept low upon formation of the SiO.sub.x film in the film forming apparatus. For example, when an SiO.sub.x film is formed by a sputter device having an Si target, if the partial pressure of moisture in the SiO.sub.x film forming chamber is high, oxygen contained in the moisture reacts with Si tending to increase the value x for the SiO.sub.x film exceeding an aimed value. This makes it difficult to control the transmittance of the SiO.sub.x film to an aimed value.
Namely, as described above, the transmittance of a light of the formed SiO.sub.x film is monitored, and the flow rate of oxygen supplied by an MFC or the like is properly controlled based on the value to form an SiO.sub.x film having a desired value for x, in which oxygen other than oxygen supplied is already present if the partial pressure of the moisture in the SiO.sub.x film forming chamber is high. Then, a margin upon controlling the amount of oxygen supplied by the MFC is extremely narrowed and, as a result, the amount of oxygen to be supplied can no more be controlled appropriately and the value x for the formed SiO.sub.x film can not be controlled stably. If the partial pressure of the moisture in the SiO.sub.x film forming chamber is excessively high, the value x is increased more than the aimed value even with no supply of oxygen at all from the MFC to the film forming chamber, thereby making it impossible to control the transmittance of the SiO.sub.x film to an aimed value.